Thermionic cathode



Feb. 23, 1937. H. "HEINS THERMIONIC QATHODE Filed NOV. 25, 1951 mmmmmmm n Patented Feb. 23, 1937 UNITED STATES reussi PATENT @FFQE THERlVIIONIC CATHODE Application November 25, 1931, Serial No. 577,234

8 Claims.

This invention relates to thermionic cathodes of the indirectly heated type.

One of the objects of my invention is to provide means for supporting the heating filament within the cathode in such a manner that the cathode may be heated very quickly to its emission temperature and the lament maintained insulated from the electron-emitting member of the cathode in a very simple manner.

The foregoing and other objects of my invention will be best understood from the following exemplifications thereof, reference being had to the accompanying drawing, wherein Fig. 1 represents a side View of one embodiment of my novel cathode with the cathode sleeve partly broken away;

Fig. 2 shows a cross-sectional enlarged view of the lower end of the cathode shown in Fig. 1;

Fig. 3 is a view similar to that of Fig. 2, showing a slightly different embodiment of my invention; and

Fig. e is a view of the cathode supported in assembled relation with the other electrodes of a vacuum tube.

In indirectly-heated thermionic cathodes, it is desirable that the cathode be raised to its electron-emitting temperature in as short a period of time as possible. The conventional type of indirectly-heated cathode now in use consists of an external sleeve carrying electron-emitting materials and a heating lament supported within this sleeve. In such cathodes it is necessary that the lament be insulated from the walls of the external sleeve order to prevent short-circuiting of the sections of the filament, and also to prevent noises of various kinds. In order to reduce the period which elapses between the` time when the filament is energized and the time when the cathode reaches a temperature at which it emits sufcient electrons to operate the device in which it is used, and also in order to reduce the expense of manufacturing such a cathode to a minimum, I reduce the amount of insulating material used to insulate the filament from the outer sleeve to a very small amount by constructing my cathode in accordance with my present invention. In previous structures the presence of a large amount of insulating material and of complex forms of this insulating material, not only lengthened the initial heating period of the cathode but also greatly increased its cost of manufacture.

In Fig. l I have illustrated a cathode comprising a thin cathode sleeve i made of some suitable material, such as, for example, nickel or iron. This sleeve is provided with a coating 2.

(Cl. Z50-27.5)

which when raised to the proper temperature becomes a good emitter of electrons. This coating preferably consists of barium or strontium oxide. The sleeve l is also provided with a conducting strip i welded thereto, which strip serves as the electrical connection to the cathode-emitting surface. Within the sleeve i is supported a heating filament i of some refractory metal, such as molybdenum or tungsten, and this filament is preferably non-inductively wound in the form of a helix in order to reduce the resultant magnetic eld on the outside of the lament to a minimum. The filament has two ends 'l and S through which heating Current may be supplied to the lament itself.

I have discovered that if such a filament as that shown in Fig. 1 is supported alone by its two ends 'i and 8 within the sleeve I, the point at which the filament Il almost inevitably comes into Contact with the sleeve i is merely the lower few turns of said filament, due to the fact that the iilament l has a tendency to expand at these lower few turns. Although I use a material for the filament li which has considerable stiifness, yet there is also some possibility of the upper portion of the iilament moving over into contact with the inner side walls of the sleeve i. Therefore, in order to insulate my filament li from the sleeve l, I have discovered that it is merely necessary to restrain the lower few turns of said filament in order to keep them from expanding into Contact with the sleeve, and to provide an insulating coating on the rest of the filament. This is done in a very simple manner by providing a short sleeve 5 which closely surrounds the lower few turns of the filament ii, and thereby keeps them from expanding into contact with the walls of the sleeve. The rest of the lament fi is provided with an insulating coating f5 which covers the entire filament i above the sleeve 5, and extendsvfor a slight distance below said sleeve 5 on the ends l and 8.

I construct my cathode preferably in the following manner. The filament e is first wound in the form of a double helix, as shown. The sleeve 5 is then slipped over the filament il, and surrounds the lower few turns of said filament. The sleeve 5 is of such a diameter that it closely surrounds these few turns which expand and initially hold the sleeve 5 in place by frictionally engaging the sides thereof. I then spray an insulating coating on the filament il above and below the sleeve 5. It will be noted that the few turns covered by the sleeve 5 are shielded from this insulating coating. However, these turns are sufliciently insulated by the sleeve 5 itself. While the insulating coating 5 may be of any suitable composition, I preferably use one which consists of a mixture of aluminum oxide with a small percentage of binding material, such as talc or powdered silica. This mixture is originally mixed with a lacquer or pyroxylin binder, such as nitrocellulose or pyroxylene dissolved in amylacetate, preferably with the addition of a small amount of alcohol. This mixture is sprayed onto the lament 4 in the manner described above, and forms a coating G of appreciable thickness. This coating is formed of such a thickness that it extends over the inner diameter of the sleeve 5 at its upper and lower ends, and when dry iirmly holds the sleeve 5 in place and prevents its moving either up or down along the lament il. The above mixtiue, after being sprayed on, is dried and then red at a temperature ranging about 1600 to G C. This results in the formation of a hard refractory insulating coating on the filament 4, which coating effectively prevents any electrical contact between the ilament and the walls of the conducting sleeve I. The larnent l itself has considerable stiiness, and the provision of the sleeve 5 gives considerable rigidity to the entire lament. The insulating coating 6 also considerably increases the stiifness of the lament so that no other support is necessary for the filament turns.

The resulting filament structure, namely the double helix filament 4, carrying the sleeve 5 and insulating coating 6, is then assembled together with the sleeve I in the device in which it is to be used. This device may be, for example, the usual vacuum tube consisting of an evacuated container having a cathode, anode, and one or more grids, in which case a structure, as shown in Fig. 4, may be used. In this ligure, I0 is the usual press of the vacuum tube. This press preferably consists of a plurality of wings extending in more than one plane. In the wings lying in one of said planes are sealed two supporting standards i I and I2. In the wings lying in another of said planes are two additional standards, not shown, supporting an anode I3. The upper ends of all of these standards project through an insulating plate I4 which is securely fastened to said standards by some such means as eyelets I5. A lower insulating plate I6, similar to plate I4, is also securely fastened to the lower ends of said standards by some such means as eyelets I1. 'Ihe control grid I8 may be formed by a fine wire wound on the two grid standards I9 and 2D, which standards are supported in the insulating plates I4 and I6, the lower end of standard 2D being electrically connected to and additionally supported by a lead-in wire 2I sealed into one of the wings of the press I0. The insulating plates I4 and I6 are each provided with a central opening which closely surrounds and engages the upper and lower ends, respectively, of the cathode sleeve I. This engagement affords some mechanical support for said cathode sleeve. The sleeve I is additionally supported by means of the conducting strip 3 being welded to one of the supporting standards II. It will be noted that by this arrangement the strip 3 not only serves as the electrical connection to the cathode sleeve, but also tends to prevent longitudinal movement of said sleeve. The insulating plates I4 and I6 accurately position the sleeve with respect to the rest of the elements, and prevent lateral movement of said sleeve. The filament i carrying its insulating sleeve 5 and coating 5 is supported independently within the cathode sleeve I by the ends l and 8 being welded to two cathode leads 22. Since these two leads lie in a plane perpendicular to the view shown in Fig. 4, but one of these leads appears in said gure. The two cathode leads 22 are also sealed in the press 9. The material of which the lament Il is made possesses suflicient stiffness and rigidity so that by welding the ends 'I and 8 to the leads 2U, sufficient support is afforded for the aforesaid lament, its coating 6, and insulating sleeve 5.

When heating current is supplied to the ends l and 8 of the lament Il, the temperature of this filament will raise to a comparatively high value in a very short time. Due to the fact that there is very little insulating material surrounding this l-ament, heat is transferred directly to the inner side walls of the sleeve I, whereupon this sleeve is also rapidly raised to its electron-emitting temperature. I have constructed such cathodes which are being used in vacuum tubes, which come up to their proper electron-emitting temperature, in from ive to eight seconds.

Instead of supporting the filament l and sleeve l independently, as is the case with cathodes of the type shown in Figs. 1 and 2, the entire cathode may be supported as a unit if so desired; in which case a construction similar to that shown in Fig. 3 may be used. Instead of using a very thin sleeve 5, I increase the thickness of this sleeve and use a sleeve 9, the outer diameter of which is the same as the inner diameter of the sleeve I. Thus the sleeve I is supported on the outside of this insulating sleeve 9 while the lower few tunis of the filament 4 extend through the inner diameter of said sleeve 9. In this manner the lament 4 is actively centered within the sleeve I, and the entire arrangement forms a unitary structure which may be mounted as a whole within the vacuum tube.

The invention is not limited to the particular details of construction, materials and processes described above as many equivalents will suggest themselves to those slnlled in the art. It is accordingly desired that the appended claims be given a broad interpretation commensurate with the scope of the invention within the art.

What is claimed is:

1. A thermionic cathode comprising a hollow member adapted to emit electrons, a double helix filament supported within said hollow member with both ends of said filament extending from one end of said hollow member, an insulating member supporting only a few lower turns of each helix of said filament, and an insulating coating on the remaining turns of said filament.

2. A thermionic cathode comprising a hollow member adapted to emit electrons, a helical filament supported within said hollow member, an insulating member surrounding a few lower turns of said lament, and an insulating coating on the remaining turns of said filament, said insulating coating extending over the upper and lower edges of said rst-named insulating member, whereby said insulating member is retained in place on said filament.

3. A thermionic cathode comprising a hollow member adapted to emit electrons, a helical lament supported within said hollow member, a thin-walled insulating sleeve surrounding the lower few turns of said helical lament, an insulating coating on the rest of the turns of said filament, the turns of said helical filament being supported within said sleeve solely by the action of said insulating sleeve, the inherent stiiness of said filament and the stiiening action of said insulating coating.

4. A thermionic cathode comprising a hollow member adapted to emit electrons, a helical lament supported within said hollow member, an insulating sleeve closely surrounding the lower few turns of said helical filament and contacting with the inner walls of said hollow member on its outer diameter, and an insulating coating on the remaining turns of said filament.

5. A thermionic cathode comprising a hollow member adapted to emit electrons, a double helix filament supported within said hollow member with both ends of said iila-ment extending from one end of said hollow member, an insulating member supporting only the lower few turns of each helix of said filament, an insulating coating on the rest of the turns of said filament, the turns of said helical filament being supported within said hollow member solely by the action of said insulating member, the inherent stiffness of said filament, and the stiifening action of said insulating coating.

6. A thermionic cathode comprising a hollow member adapted to emit electrons, a double helix filament supported within said hollow member with both ends of said filament extending from one end of said hollow member, an insulating member supporting only the lower few turns of each helix of said filament, and a thin insulating coating on said l-ament, said insulating coating cementing said insulating member in position on said filament, said filament together with the insulating coating thereon being spaced from said hollow member, the space between the interior of the thermionically-active portion of said hollow member and said lament carrying said coating being free of intervening solid material.

7. A thermionic cathode comprising a hollow member adapted to emit electrons, a helical lilament supported within said hollow member, and an insulating sleeve closely surrounding and. directly engaging only the lower few turns of said helical filament.

8. A thermionic cathode comprising a hollow member adapted to emit electrons, a helical iilament supported within said hollow member, and an insulating sleeve closely surrounding and directly engaging only the lower few turns of said helical filament, said insulating sleeve and the inherent stiffness of said filament being the sole supporting means for the turns of said helical filament within said sleeve.

HAROLD HEINS. 

